Variability ofCylindrocladium crotalariaeResponse to Resistant Host Plant Selection Pressure in Peanut

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Abstract
Seventy-nine isolates of C. crotalariae originally obtained from plants of the resistant peanut cultivar NC 3033 and 11 isolates obtained from susceptible plant species including peanut were tested by inoculating replicates of the susceptible cultivar Florigiant and resistant cultivar NC 3033. The mean virulences of isolates from the susceptible hosts did not differ significantly from those of isolates from the resistant host. When disease data were fitted to a pathogen virulence model, however, differences were noted among isolates from the resistant host. As isolates became adapted to NC 3033, they tended to become less adapted to ''Florigiant''. Following only 1 cropping cycle of the resistant host, virulence specific for ''NC 3033'' increased in a previously nonselected pathogen population. A potential exists for race development in C. crotalariae even though corresponding resistance in the host apparently was quantitatively inherited.